Mesozoic Volcanic and Hypabyssal Rocks

Several ranges within the EMNSA contain volcanic and volcaniclastic rocks, intercalated
sedimentary rocks, and related hypabyssal rocks of Triassic and (or) Jurassic age.
Stratigraphic sequences, in varying degrees of preservation, are exposed in four areas: the
Mescal Range, the Old Dad Mountain-Cow Hole Mountains-Soda Mountains area, the New York
Mountains, and the Providence Mountains. In a few other areas, metamorphosed or
hydrothermally altered Triassic and (or) Jurassic volcanic, hypabyssal, and sedimentary rocks
are present as small pendants in Jurassic or Cretaceous plutons, or as slivers in fault zones.
Among these small relicts, Jurassic rocks are probably more common than Triassic rocks.
Owing to common metamorphism or alteration, and to lack of study, little is known about the
petrology and geochemistry of Triassic and Jurassic volcanic rocks in the EMNSA.
A sequence of diverse volcanic and sedimentary rocks, more than 3 km thick, in Old Dad
Mountain and the Cow Hole Mountains consists of interbedded intermediate to silicic lava flows
and flow breccias, quartz arenite, sandstone and siltstone, sedimentary breccia and
megabreccia, silicic ignimbrite, and other minor rock types (Busby-Spera, 1988; Busby-
Spera and others, 1989; Marzolf, 1983; 1988; 1991). U-Pb zircon ages of some of the
volcanic rocks indicate that this sequence is approximately 170 Ma, which is Middle Jurassic
according to the geologic time scale of Harland and others (1989). A generally similar sequence
of rocks is present in the Soda Mountains, near the west edge of the EMNSA (Grose, 1959).
The quartz arenite units in the Mescal Range, Old Dad Mountain, and the Cowhole
Mountains are in part eolian. Until recently, these quartz arenites were generally correlated
with the Early Jurassic (Peterson and Pipiringos, 1979) eolian Aztec Sandstone of the southern
Great Basin and Navajo Sandstone of the Colorado Plateau. However, the U-Pb ages cited above
indicate that the Jurassic quartz arenites in these ranges, and probably others in the EMNSA,
may correlate with the Carmel Formation or the Entrada Sandstone of the Colorado Plateau.
Poor age constraints for the Colorado Plateau units permit either correlation.
In the Providence Mountains, intermediate to silicic volcanic, volcaniclastic, and
hypabyssal rocks, in part intensely altered, have been mapped by Miller and others (1985) and
Goldfarb and others (1988). The hypabyssal rocks typically have granitic textures. These
igneous rocks in part overlie the Moenkopi Formation, and are probably Triassic and (or)
Jurassic in age (Walker, 1987). In some places, the volcanic rocks contain intercalated
conglomerate and siltstone.

In the New York Mountains, a sequence of metamorphosed volcanic rocks approximately
250 m thick overlies the Moenkopi Formation, and is in turn overlain by a metasedimentary
unit approximately 70 m thick (Burchfiel and Davis, 1977). The volcanic rocks are silicic in
composition, include breccia or agglomerate, and contain subordinate intercalated metasiltstone
and, near the base of the unit, metaconglomerate. The metasedimentary unit comprises
siltstone, conglomerate, and tuffaceous sandstone and siltstone. The conglomerate beds contain
clasts derived from the underlying volcanic unit. These two units could be either Triassic or
Jurassic in age; the latter is perhaps more likely. The metavolcanic rocks are generally
schistose or mylonitic; this fabric probably is largely inherited from original welded tuff
textures. Metasedimentary lithologies range from argillite to schist. Both the volcanic and
sedimentary units in the New York Mountains contain metamorphic biotite.
In the Mescal Range, a unit of crossbedded arenitic sandstone approximately 250 m thick
contains dinosaur tracks, the only dinosaur tracks known in California (Reynolds, 1983). It is
overlain by a sequence, approximately 200 m thick, of basaltic, dacitic, and rhyolitic flow
breccias and lava flows (Hewett, 1956; Fleck and others, in press). These volcanic rocks have
not been studied in detail, but are dated by K-Ar and Rb-Sr as Early Cretaceous, about 117 Ma
(Fleck and others, in press), and therefore differ in age as well as composition from Jurassic
volcanic sequences in other parts of the EMNSA.
Jurassic and Cretaceous Plutonic Rocks